Automatic frequency control circuit



March 4, 1941.

D. E. FOSTE R AUTOMATIC FREQUENCY CONTROL "CIRCUIT Filed July '7, 1938 21/0 0571 DISCRIMINATOR 70/21 1.5 & 704/? AMP 057: AMPL. P1 -Erw0A AFC- f0 1 6L 6- 6/ L T P .9 10cm M M ,3 ,2 OSCILLATOR INVENTOR- DUDLEY E. FOSTER A TTORNEY.

Patented Mar. 4, 1941 UNITED STATES PATENT OFFICE AUTOMATIC FREQUENCY CONTROL CIRCUIT Dudley E. Foster, South Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 7, 1938, Serial No. 217,844

1 Claim. (01. 250-20) My present invention relates to automatic freticularity in the appended claim: the invention quency control circuits, and more particularly itself, however, as to both its organization and to a novel method of, and means for, supplemenmethod of operation will best be understood by tally adjusting the frequency of an oscillator emreference to the following description taken in 5 ploying non-reactive tuning elements. connection with the drawing in which I have One of the main objects of my present invenindicated diagrammatically a circuit organization is to provide a method of adjusting the tion whereby my invention ma be carried into frequency of an oscillation circuit of the type effect. employing resistance tuning; the frequency ad- Referring now to the accompanying drawing,

'10 justment being auxiliary to the resistance tunthere is shown a superheterodyne receiver sysing, and being accomplished by varying the magtem of a generally conventional type, except for nitude of negative resistance in circuit with the the construction of the local oscillator network. resistor used for main tuning control. The receiver comprises the usual signal collector,

Another important object of the invention may adapted to collect signals in the broadcast band be stated to reside in the provision of an oscilof 550 to 1500 k. c., feeding collected signal en- 15 lator of the type using an electron discharge deergy to the tunable radio frequency amplifiers vice havin circuit elements associated therewith (not shown). The radio frequency amplifier may which comprise negative and positive resistance comprise one or more stages of amplification, elements in a balanced state, the positive reeach stage including a tube provided with a tunzo sistance element being adjustable in value to efable input circuit. The usual first detector netfeet substantial frequency changes of the oscil- Work is supplied with amplified signal voltage lator, and the negative resistance element being by the tunable radio frequency amplifier, and the variable at selected values of the positive reintermediate q e y (I. F.) outp of the sistance element to produce small changes in first detector is amplified by the conventional .25 frequency. I. F. amplifier. The amplified I. F. energy may .25

Another object of my invention is to provide then be impressed upon a combined second dea superheterodyne receiver which has a local osmotor-freq y discriminator network of the cillator of the type employing resistance of adtype disclosed by S. W. Seeley in application justable value for tuning over a wide range of Serial No..4 fi d 1935 granted June 0 frequencies; frequency shifts in intermediate fre- 21, 1938 as U. S. P. 2,121,103. In general, the quency (I. F.) energy from an assigned frepresent receiving circuit may be of the same type quency value being employed to produce a varishown in the said Seeley patent, particularly Fig. able direct current voltage which. is applied to 4 thereof; the construction of the local oscillator the oscillator thereby to vary the oscillator frenetwork and the freque Control mechanism quency at selected values of the said resistance. being difie in the present Case. 35

Still another object of my invention is to pro- Briefly, it may be stated that the function of vide a superheterodyne receiver of the push butthe second detector-discriminator network, as ton type; the local oscillator using a plurality is well known to those skilled in the art, is to of stable adjustable resistors in circuit with a derive io energy nd a r q -dep n n regeneratively coupled tube, and each resistor direct current voltage from the I. F. energy. The

representing a desired oscillation frequency; a discriminator network can be entirely independfrequenc discriminator network, responsive to ent of the second detector circuit. As explained I. F. energy shifts from -an assigned frequency in said Seele patent, the direct current voltage value, producin direct current voltage, and the output of the discriminator varies in polarity and latter voltage being applied to the oscillator grid magnitude with the sense and amount respectiveto vary the value of negative resistance in cirly of frequency departure of the I. F. energy from cuit with a selected one of said resistors thereby the assigned I. F. value. The variable direct to maintain the assigned frequency value. current voltage, or AFC bias, is transmitted over St l other lec s of the invention are to imlead ID to the local oscillator tube 3.

- prove generally the simplicity d fic ency of The local oscillator comprises the tube 3 which 50 automatic freque cy control circuits for radio includes at least the cathode IS, the control grid receivers, and more especially to provide such l5 and plate H. The plate is regeneratively coucontrol circuits in an economical manner. pled to the grid l5 by means of the inductive The novel features which I believe to be charcoupling M between the plate coil 1 and the grid '55 acteristic of my invention are set forth in parcoil 2. Direct current voltage of the proper positive value is applied to plate ll through coil I, whereas coil 2 is connected between the grounded cathode l6 and the direct current blocking condenser 9. The plate i i is further connected to ground through a path which includes a fre quency-dependent impedance network 4-5, a condenser 1, and any one of a plurality of resistors 6-6'--6. The impedance d--5 comprises the condenser E and shunt resistor 4, and the function of this network will be explained at a later point. The resistors 665" function as the main tuning means for the local oscillator network. Each of these resistors has a magnitude such that when in circuit with the network 45, the local oscillator. is tuned to a desired" local oscillation frequency. In order to select the proper tuning resistor each of the shunt resistors is connected to ground through a pushbutton switch; hence, resistors fi66 have associated with them switches l2, l3 and 44 respectively,

It will be understood that closing any of these push-button switches results in adjustment of the local oscillator to a desired oscillation frequency. The local oscillation energy is impressed on the first detector network in any desired manner, one well known arrangement being shown in the drawing by illustrating the grid l5 as being connected through condenser II to the first detector network. Of course, the frequency of the local oscillations will differ from the signal frequency impressed on the first detector by the frequency value of the I. F. energy. It should be understood that the tunable circuits of the radio frequency amplifier and first detector may also include push-button switches to select appropriate tuning condensers so as simultaneously to vary the signal circuits to differ carrier frequencies of the broadcast band. In such case the switches l2, l3 and [4 of the local oscillator network will be m chanically operated with the signal circuit selector switches so that at different adjustments of the oscillator and signal circuits the I. F. energy will have substantially the assigned I. F. value. The function of the AFC circuit is to correct for any deviation from the assigned I. F. value, should the oscillator or signal circuits shift in frequency. In place of the plurality of resistors 66'6" there may be employed a single adjustable resistor.

The AFC bias is applied to grid l5 by lead H] through the grid leak resistor 8. The variation in bias of grid [5 is converted into a change in gain of 'tube 3; the change in gain, in turn, is translated into a frequency shift of the oscillator network. The polarity of the AFC bias determines the direction in which the oscillator frequency is varied to compensate for the I. F. energy in Wireless Engineer for Sept. 1937 on page 469; essentially the networkemploys negative and positive resistance to produce oscillations of 'a desired frequency. The regeneratively coupled tube 3 provides the negative resistance; resistor 6 provides positive resistance whose magnitude is adjusted to vary the oscillation frequency. By varying the gain of tube 3 the value of negative resistance can be adjusted. Hence at selected values of positive resistance 6, the AFC bias can be used to secure small frequency adjustments in order accurately to tune the oscillator to that frequency which will cause the proper I. F. value to be produced. The grid |5-has a normal negative bias value by virtue of grid current flow due to tube 3 being in an oscillator circuit. It is desirable to maintain grid l5 at such a bias value as to cause the frequency shift to be substantially symmetrical with variations in applied discriminator potential about a mean value.

The following detailed considerations of the functioning of'the oscillator network will make the operation clearer. In a circuit consisting of a pure negative resistance, which is a function of frequency, and a positive resistance in series therewith, oscillation will occur when the net resistance around the circuit is zero. The frequency of oscillation is that frequency which will make the negative resistance component equal to the positive resistance component. Therefore, changing the positive resistance component will change the oscillation frequency. Now the re-' quired negative resistance which is a function of frequency can be secured by connecting across a negative resistance which is independent of frequency a reactance, and in series with that combination a reactance of opposite signs hunted by a resistance of approximately the same magnitude as the negative resistance. Therefore, small changes of the magnitude of the negative resistance will likewise afiectthe oscillation frequency.

In the circuit shown inductance l is coupled to coil 2 and through blocking condenser 9 to the grid of tube 3. The output of tube 3 is connected to coil I so an oscillation circuit is formed. The plate-cathode impedance of tube 3 forms negative resistance in shunt with coil l In series with this combination is condenser 5, and positive resistance 4 in shunt therewith, approximately equal to negative tube resistance. This combination l345 forms a negative resistance which is a function of frequency, as resistance 4 and the negative resistance of 3 are approximately equal and each is approximately equal to where L is the inductance of coil 1 and C is the capacity of condenser 5.

Now if resistance '5 is zero, the frequency is or the normal frequency of an oscillation circuit composedof inductance L of coil I and capacity C of condenser 5. If resistance 6 equals resistance'4, the oscillation frequency is theoretically infinite, but is actually limited to some finite value by the distributed capacity of coils. Decreasing negative resistance a small amount so that the relationship that negative resistance is equal to resistance 4 and to y still holds approximately, will likewise cause frequency increase. If the AFC bias is applied to the grid of tube 3, through a dc. amplifier if necessary to obtain a low impedance source since tube 3 being an oscillator will draw some grid current, it will change the operating transconductance of tube 3, and hence the magnitude of negative resistance between plate and cathode. If the grid of 3 is made more negative the transconductance of tube 3 will decrease, and the negative resistance will increase thereby resulting in a decrease of oscillation frequency, and vice versa.

From the foregoing considerations and relationships it is believed that those skilled in the art will be able to construct an AFC circuit for a superheterodyne receiver wherein frequency shifts of the I. F. energy from an assigned I. F. value are compensated for. It will be observed that in this oscillator network automatic frequency shift correction is secured without using an auxiliary frequency control tube; the existing oscillator network is provided with means for providing the correction tuning adjustments. Hence, the present arrangement simplifies the frequency control circuit in that the oscillator tube acts both as a primary tuning means by variation of the positive resistance 6, and as an AFC device 'by virtue of the variable bias applied to the oscillator grid.

It is also pointed out that in receivers employing push-button tuning the present invention has great advantage. In such push-button sets it is clifficult to secure a stable adjustable capacity, and it is costly to secure a stable oscillator circuit by a multiplicity of adjustable inductances where the station selection is by selection of pre-set tuned circuits. Stable adjustable resistances are procurable. Since in resistance tuning a tube is used for attaining proper phase relationships, the advantages of employing adjustable stable resistance for resistance tuning of the oscillator, with AFC secured by varying the gain of the oscillator tube, may readily be secured.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claim.

What I claim is:

In a superheterodyne receiver of the type including an intermediate frequency transmission network and a discriminator circuit adapted to derive a direct current voltage from the intermediate frequency energy whose polarity and value is a function of the sense and magnitude of frequency departure of the intermediate energy from an assigned intermediate frequency value, a local oscillator of the type whose frequency is a function of positive and negative resistance and which comprises a tube having at least a cathode, control grid and anode, means for regeneratively coupling the anode and grid to provide said negative resistance, variable resistive means adjustable to different values in circuit with the tube to provide said positive resistance, means for varying said resistive means to tune the oscillator to a desired frequency, and means for applying said direct current voltage to the oscillator tube grid thereby to vary the magnitude of said negative resistance sufficiently to adjust the oscillator frequency for correction of oscillator frequency inaccuracy at a selected positive resistance value.

DUDLEY E. FOSTER. 

